In a multi-cylinder engine, exhaust pipes extend from the exhaust ports of the cylinders, and these exhaust pipes are brought together in an appropriate manner. The member used to bring the pipes together is an exhaust pipe manifold. Main exhaust pipes (usually one or two) extend from the exhaust pipe manifold and pass under the floor of the vehicle body to the rear of the body.
The exhaust pipe manifold inevitably has a complicated structure. In view of this, assemblies for fitting together a plurality of exhaust pipes are known, such as the one disclosed in Japanese Patent Application Laid-Open Publication No. 2000-240450 (JP 2000-240450 A). FIGS. 8 through 10 hereof show the assembly disclosed in JP 2000-240450 A.
Referring to FIG. 8, an assembly 100 is comprised of a perforated cap 101, a choke pipe 102 connected to the perforated cap 101, and a flange 103 provided at the exit of this choke pipe 102.
The choke pipe 102 is a cylinder in which the entrance has a large diameter, the exit has a small diameter, and the inside diameter decreases gradually.
The perforated cap 101 shown in FIG. 9 is comprised of a flat surface 104, a skirt 105 bent downward from the edge of the flat surface 104, a plurality (four in this example) of holes 106 formed in the flat surface 104, and sockets 107 formed by extending the edges of the holes 106 upward by means of burring. The distal ends (the bottom ends in the drawing) of exhaust pipes 108 are inserted into the sockets 107 and are fixed in place by welds 109, as shown in FIG. 8.
Since the skirt 105 is displaced further outward (to the left in the drawing) than the sockets 107 as shown in FIG. 10, pockets 111 can be formed in the portions enclosed by the bottom ends of the exhaust pipes 108, the top end of the choke pipe 102, and the skirt 105.
First currents 112 flowing along the internal peripheral surfaces of the exhaust pipes 108 form eddy currents in the pockets 111. These eddy currents increase pressure loss and are a hindrance to the flow of exhaust gas.
Second currents 113 slightly nearer to the center than the first currents 112 strike the internal surface of the choke pipe 102 and move in irregular fashion. The irregular movement creates turbulence and impedes the smooth flow of exhaust gas.
A smoother flow of exhaust is desired in the areas where the exhaust pipes are fitted together, and there is therefore a demand for a structure that does not cause eddy currents or turbulence.